Honeybee queens exposed to pesticides transfer part of the toxic burden into their eggs, according to new research from the University of California, Davis, published August 21, 2026, in Current Biology. The study provides the first evidence in honeybees of this survival strategy, known as maternal offloading — a defense that protects the queen while potentially putting her offspring and the colony at risk.

The finding changes how practitioners and beekeepers should think about pesticide exposure in apiaries: contamination does not stop at the worker level, and it does not necessarily stay in the queen's body either. It moves into the next generation.

How the study worked

Most previous honeybee toxicology research focused on worker bees. This study, led by UC Davis in collaboration with Lawrence Livermore National Laboratory (LLNL) and the USDA's Agricultural Research Service (USDA-ARS), instead tracked where chemicals ended up throughout the colony — in the queen, her ovaries, her eggs, and the wax.

The researchers built "nanocolonies": small experimental systems designed to reproduce key functions inside a hive. Each consisted of a conical plastic container with a netted bottom holding one queen and 60 worker bees. The bees received pollen, water, and food contaminated with the pesticide methyl parathion, which the researchers tagged with a low-level radioactive marker to follow its movement through the colony.

Detection relied on biological accelerator spectrometry (BioAMS) at LLNL, which can trace radioactive markers at extremely small concentrations. "The pesticide concentrations we used were not lethal and were environmentally relevant to that seen in nature," said Bruce Buchholz, an LLNL scientist and author on the paper.

Worker filtration breaks down over time

The results quantify how far social buffering stretches — and where it fails. On the first day of exposure, worker bees filtered out 95% of the pesticide and transferred it into the honeycomb. By day 10, that figure had dropped to 86%.

"In our study, pesticides began to accumulate in queens over time, suggesting that worker filtration capacity can be overwhelmed," said Angela Encerrado-Manriquez, the paper's lead author and a recent Ph.D. graduate from UC Davis. "When this happens, queens have their own defense. Maternal offloading allows them to shunt the toxic burden to their eggs."

"In order to protect herself, the queen bee offloads these chemicals into her eggs to get rid of them," said Sascha Nicklisch, the paper's senior author and an associate professor in UC Davis's Department of Environmental Toxicology. "No one has shown this in honeybees before."

Why it matters at the hive and in the field

Honeybee queens lay 1,500 to 2,000 eggs per day, and she is the only hive member capable of producing the next generation of workers. Honeybees pollinate roughly one-third of the world's food crops, so colony losses carry consequences for agricultural productivity and food security.

The findings suggest pesticides may keep accumulating in a colony even after exposure begins, especially once workers can no longer remove contaminants effectively. For beekeepers, growers, and integrated-pest-management planners, the practical concern centers on periods when bees actively forage or colonies are expanding — precisely when the pressure on the queen's reproductive system may build unnoticed.

"When pesticides accumulate to the extent that the queen bee has eggs that are so loaded they may no longer develop properly, there could be a tipping point," Nicklisch said. "There may be a slow creeping effect of chemical accumulation that will contribute to delayed colony collapse."

Open questions

Researchers still do not know how long queens can continue transferring contaminants into their eggs, what the long-term consequences are for the colony, or whether the process differs depending on the pesticide involved. Those questions are targets for future research.

Julia Fine and Eliza Litsey of USDA-ARS and David Baliu-Rodriguez, Sean Leonard, and Bruce Buchholz of LLNL contributed to the work. Funding came from the USDA's National Institute of Food and Agriculture and Non-Assistance Cooperative Agreement program, the PAm-Costco USA Scholarship program, and the University of California National Laboratory Fees Research Program, with LLNL work performed under a U.S. Department of Energy contract.

Future studies tracking offloading duration across different pesticide classes will determine whether maternal offloading is a sustainable defense or a pathway to delayed colony failure.